In coating by electrodeposition, the electrodeposition coating materials preferably employed are those which contain, as binder, cationic, amine-modified epoxy resins as a water-soluble based resin component, and polyisocyanates blocked with alcohols and/or amines as a crosslinking agent.
For reasons of corrosion prevention, it is currently preferred to employ epoxy resins synthesized on the basis of bisphenol A, which are reacted with amines in protic solvents such as alcohols and glycols and/or aprotic solvents such as ketones and aromatic compounds in order to render them soluble in water.
It is these bisphenol A-based epoxy resins, in particular, which are outstandingly suitable for corrosion prevention and are therefore preferably employed for the synthesis of amine-modified epoxy resins.
In order to prepare the individual binder components, for reasons of viscosity or reactivity, it is necessary to employ solvents. For instance, in the synthesis of the base resins, protic (e.g., alcohols, glycols) and aprotic (e.g., ketones, esters or aromatic compounds) solvents are used, and aprotic solvents are used in the synthesis of the crosslinking component.
However, as is known, as the solvent content of cathodic electrodeposition (CED) coating baths increases, the throwing power (coating of cavities) is impaired, instances of excessive coating occur, and an increased quantity of solvent passes into the waste water and/or the waste air. For these reasons the solvents have to be stripped off from the binders before and/or after dispersion in water, for example, by distillation or ultrafiltration. These methods result in a portion of the solvents remaining in the dispersions, and, especially during so-called "stripping" in the aqueous phase and during ultrafiltration, relatively large quantities are obtained of a solvent-water mixture which has to be disposed of.
In DE-A 43 14 297, improved synthetic resin dispersions are described which give coatings having a good resistance in, for example, the salt spray test.
A parameter which requires improvement in the unsaturated monomers specified in DE-A-43 14 297 and which are used as solvents for the synthesis of binders, is their compatibility or "interaction" with the polar epoxy-amine adducts based on epoxy resins.
This incompatibility is manifested, for example, in the increased viscosity a) of the epoxy-amine adducts prepared in monomers (especially aromatic vinyl compounds such as styrene) and b) in the melt viscosity during the baking procedure.